The present invention relates to the fabrication of hollow structures and, more particularly to a novel method for fabricating components for rocket engines, gas turbine engines and the like which have hollow structures.
A conventional method for manufacturing hollow structures includes the step of forming the desired interior configuration in a side portion of a workpiece either by machining the workpiece to form the shape of the hollow interior, or by casting the workpiece with the desired hollow interior shape formed in the workpiece side portion. A filler or mandrel is then machined or otherwise formed which corresponds substantially to the shape of the hollow interior; the mandrel is placed in contacting mated relation with the interior configuration formed in the workpiece, to provide a core. A close-out layer of material or hot wall, as it is sometimes referred to, is applied over the mandrel and connected to the boundary edges of the workpiece around the mandrel to close the structure over the mandrel and to complete the workpiece. Access to the interior mandrel or filler material must be available after the close-out layer is deposited. The mandrel or filler material may then be dissolved by chemical etching and the like to remove it from the interior of the workpiece and thereby provide a hollow structure. The etchant must only attack the filler material and not the workpiece and close-out material. The materials used for the workpiece, close-out and mandrel are therefore restricted to be compatible with the etching operation.
Conventional methods for forming structures with hollow interiors, similar to the method described above, have several disadvantages. Intricate hollow interior structures, such as a re-entrant angle or the surpentine interior configuration of a gas turbine engine blade, vane, or the like, cannot be easily formed using the above-described method; the method is also not suitable for making numerous, small voids or hollows in a large component, such as cooling channels in the wall of a combustion chamber of a liquid propellant rocket engine, scramjet engine or the like. A precisely machined mandrel would have to be formed and inserted into each one of a multiplicity of channels formed around an interior wall of the combustion chamber before a close-out layer of material is deposited, necessarily a very laborious and time consuming operation. The process is further complicated in that the channels are not necessarily of uniform dimensions throughout their entire extent.
Another disadvantage of current methods is that the mandrel is typically an iron or titanium tube; an iron tube is typically dissolved with hot phosphoric acid and a titanium tube is typically dissolved with room temperature hydrofluoric acid. Thus the mandrel is sacrificed and the waste must be properly disposed of.
The chemical ething or leaching operation of conventional methods to remove the mandrel material is typically extremely slow, and disposal of the resulting waste material can be expensive and is an environmental concern. The choice of materials for the workpiece, close-out and mandrel is thus restricted so that the chemical etchant used will only dissolve the mandrel material.